The present invention relates to a tape driving system for use in recording and/or reproducing apparatus, and more particularly to an apparatus capable of reproducing a tape on which video signals have been recorded, wherein the tape travels or shifts intermittently in a manner such that good quality slow-motion reproduction of the video signal can be carried out.
In a helical scanning type of video recording and reproducing apparatus, the video signals are successively recorded by one or more rotating heads on tracks oblique to the longitudinal direction of the tape, with, for example, one field being recorded per track. For slow motion reproduction of these recorded video signals, the tape is driven at a speed which is slower than that used in the recording mode (or normal reproducing mode). Accordingly, in reproduction, the rotating heads repeatedly trace the respective tracks a number of times, whereby the reproduced picture has a slower motion than it would have if reproduced in a normal reproducing mode. It is in this way that slow-motion reproduction is achieved. Moreover, when the tape is stopped, a single track is repeatedly traced by the rotating heads to reproduce a still picture.
In the slow-motion reproducing mode or the still picture reproducing mode, the rotating heads rotate at the same speed as in normal reproduction mode, while the tape is driven at a speed slower than that in a normal reproducing mode or is stopped completely. As a result the inclination angle of the track traced by the rotating head on the tape in slow-motion and still picture reproduction mode differs from the inclination angle of the track in a recording mode (or normal reproducing mode) and this difference in inclination gives rise to tracking deviation.
In reproduction, when the rotating heads deviate from the signal tracks on the tape, a noise bar is generated in the reproduced picture. In the conventional slow-motion reproduction, the tape is continuously driven at a speed slower than the normal traveling speed at the time of a normal reproducing mode. The position at which the rotating heads deviate from the tracks undergoes successive shifts. For this reason, the position of the noise bar continuously shifts in the reproduced picture, (from top to bottom, for instance) and the noise therefore impairs the quality of the entire reproduced picture.
Similarly, in the case of still picture reproduction, depending on the position at which the tape stops, the noise bar will be adversely generated at the center or some other conspicuous part of the picture screen. In this case, the operator must manipulate the reel to rotate it slightly and cause the tape to shift a minute distance so that the noise bar becomes concealed on the top or bottom of the picture screen. However, in the cassette-type recording and reproducing apparatus reduced to practice in recent years, the reels are positioned within the cassette. Thus, it is impossible for the operator to shift the tape by a manual operation.
A DC motor is generally used for rotating the capstan so as to make it possible to drive the tape at different speeds. When the DC motor is to be rotated slowly for slow speed tape travel, the voltage applied to the DC motor is lowered to a potential which is less than the potential used at the time of normal rotation. However, when the voltage applied to the motor is lowered, the rotational torque of the motor decreases, and as a result the tape cannot be driven stably.
Moreover, when the DC motor is to be rotated at a slow speed, it is necessary to overcome starting friction by first applying to the motor a higher voltage, relative to the voltage required to obtain the desired low speed of rotation. Then, after the motor has started the voltage is reduced to the value required for slow speed rotation. However, the raising and lowering of the voltage applied to the DC motor to change the speed of rotation gives rise to hysteresis thus introducing a complex factor into rotation speed control and adjustment.
In addition, some video recording and reproducing apparatus have two motors for driving a capstan, one motor being used for normal reproduction (for normal speed rotation) and another motor for slow motion reproduction (for slow speed rotation). At the time of slow motion reproduction, the rotation of the motor for slow motion reproduction is transmitted to the capstan. This apparatus eliminates the above described difficulties but requires a more complex mechanism which must be capable of switching between the two motors.
Moreover, another known apparatus transmits the rotation of a single motor through a reduction changeover mechanism to the capstan a slow-motion reproducing mode. This apparatus, however, requires a reduction changeover mechanism, thus increasing the complexity of the mechanism.
Furthermore, if these conventional apparatus are to be contstructed so as to allow the operator to freely and continuously vary the speed of rotation of the capstan, their mechanisms must be made even more complex.